Gas turbines play a significant role in a number of applications, such as aircraft propulsion, marine propulsion, power generation and driving processes, such as pumps and compressors. Typically, a gas turbine includes a compressor, a combustor and a turbine. In operation, air is fed into the system where it is compressed by the compressor and mixed with fuel in the combustor. The compressed air and fuel mixture are then burned within the combustor to cause an expansion of the air flow, which is responsible for driving the turbine.
Combustion liners define the interior volume of the combustor and serve to protect the combustor casing and remaining engine parts from the extreme temperatures present within the combustor. In order to reduce NOx emissions and extend the longevity of engine components, it is desirable to use a portion of the compressed air exiting the compressor for cooling various combustor components, including the combustion liner.
Combustion liners often include a venturi throat region that is used to influence the flame location within the combustor and to stabilize the combustion flame. In such arrangements, a lowering of the NOx emissions is achieved by lowering peak flame temperatures through the burning of a lean, uniform mixture of fuel and air. Uniformity is typically achieved by pre-mixing fuel and air in the combustor upstream of the venturi and then firing the mixture downstream of the edge or apex of the venturi into the secondary combustion chamber. The venturi configuration, by virtue of accelerating the flow preceding the throat portion, is intended to keep the flame from flashing back into the pre-mixing region or primary combustion chamber, upstream of the venturi throat. The flame holding region, upstream of the venturi, is also required to be cooled under certain operating conditions. Since flames can reside on each side of the venturi, both sides need to be cooled. However, if the flow of cooling air released downstream of the venturi is too large or is released too close to the venturi, it may adversely affect combustion performance.
U.S. Pat. No. 5,117,636 shows a cooling passageway within the venturi that is extended downstream from the venturi apex or throat, such that the cooling air does not re-enter the flame holding zone within the secondary combustion chamber. It is intended that the functions of flame holding and venturi cooling are decoupled. U.S. Pat. No. 6,430,932 shows a cooling chamber within a venturi that directs the cooling flow counter to the combustion flow and dumps cooling air upstream of the venturi.
It has been determined that the heat load on the upstream and downstream sides of the venturi are different and the dumping of cooling air near the venturi on either side has an effect on the operation of the combustor. In the prior designs, where the cooling of both sides of the venturi are linked, there has not been a balancing of the heat pickup within the venturi walls nor has there been an accommodation made for pressure drop within the cooling passageway.